As this type of electric storage device, those described in Patent Document 1 (Japanese Patent Laid-Open No. 2007-53002) and Patent Document 2 (Japanese Patent Laid-Open No. 2001-38475) are known. The lithium secondary battery cell 1 (electric storage device) described in Patent Document 1 includes: an electrode unit 10 (electrode assembly) having a pair of current-collecting-terminal connection portions 122a and 142a (protrusion portions) in which an end portion of each of a positive electrode sheet 12 (positive electrode foil) and a negative electrode sheet 14 (negative electrode foil), which are wound around into a flat shape, protrude from a side end of another one in a laminated state; a positive current collecting terminal 22 (current collector) joined to the current-collecting-terminal connection portion 122a of positive electrode by ultrasonic welding; and a negative current collecting terminal 32 (negative collector) joined to the current-collecting-terminal connection portion 142a of negative electrode by ultrasonic welding. Further, a battery cell (electric storage device) described in Patent Document 2 includes: a winding-type electrode assembly 1 (electrode assembly) having a pair of joint portions (protrusion portions) in which an end of each of a positive electrode plate 11 (positive electrode foil) and a negative electrode plate 12 (negative electrode foil) which are wound into a flat shape, protrudes from a side end of another one in a laminated state; and a current outlet terminal 2 and a buffer plate 4 (metal material) each of which is joined by ultrasonic welding in a state of being laid on top of the front or back side of each joint portion.
As shown on the upper sides of FIGS. 10A and 10B, an electrode foil of opposite side (not shown) and a separator (not shown) as an insulation material lie between electrode foils 8. As a result, a protrusion portion 8A of an electrode assembly obviously has a thickness larger than a dimension of the thickness of the electrode foil 8 multiplied by the number thereof. In Patent Document 1, as shown in FIG. 10A, ultrasonic welding is conducted in state that a contact portion 3A of a current collector is placed on an anvil 30 which is a pedestal portion of a welding machine, the protrusion portion 8A is placed on the contact portion 3A, and a tip (or a horn) 31 of the welding machine is held in direct contact with the protrusion portion 8A. In such a condition, the protrusion portion 8A bends abruptly and significantly at a boundary between a portion which is pressurized by the tip 31 and a portion which is not pressurized by the tip 31. If, in that state, the protrusion portion 8A is subjected to ultrasonic vibration, it becomes more likely that unfavorable conditions such as cracking and breakage thereof occur, above all, on the front face side of the protrusion portion 8A. Moreover, if such an unfavorable condition occurs, the conductivity is adversely affected, which may lead to adverse effects on the ability (performance) as the secondary battery cell such as durability and reliability.
It is conceivable, as a countermeasure, to hold the tip 31 in contact with the protrusion portion 8A via a metal material 13 as with Patent Document 2. However, when the metal material 13 has a high rigidity, such as when the thickness of the metal material 13, which has a uniform thickness, is sufficiently large, the metal plate 13 which is pressurized by the tip 31 will pressurize the protrusion portion 8A without being deformed as shown in FIG. 10B. If that case, stress concentration occurs at a portion corresponding to an end of the metal material 13 in the protrusion portion 8A, and unfavorable conditions such as cracking or tearing become more likely to occur in a portion surrounded by a circle in FIG. 10B. That is, the portion where unfavorable conditions occur has just moved from an end of the tip 31 to an end of the metal material 13, and the above described unfavorable conditions themselves are still likely to occur.